The invention relates to a power switching cell and to a process for fabricating this cell.
More precisely, the invention relates to a switching cell comprising:                at least two power components each exhibiting a first face equipped with at least one electrical bonding terminal and one opposite second face equipped with at least one other electrical bonding terminal, these components forming a chain of components electrically linked in series by way of at least one intermediate bond, each intermediate bond electrically connecting the second face of the previous component to the first face of the next component in said chain, and        a dielectric substrate inside which are incorporated said at least two components.        
Such switching cells are, for example, frequently used to make inverters intended to power electric motors used for pulling trains. In such applications, the switching cell must be capable of switching currents greater than a thousand amperes and of withstanding voltages of greater than five thousand volts. To achieve such performance and in particular to resist such voltages, each switching cell is made from several elementary interrupters such as IGBT transistors, linked together in series.
Moreover, today, the CPES (Center for Power Electronics System (CPES), Virginia Tech., 657 Whittemore Hall (0111), Blacksburg, Va. 24061) has disclosed a process for incorporating transistors working at less than 600 volts, inside a substrate. For example, in the article “Embedded Power—An Integration Packaging Technology for IPEMs” by the authors Zhenxian Liang, Fred C. Lee and G. Q. Lu, a copy of which may be obtained from the CPES, a first transistor IGBT1 and a second transistor IGBT2 linked in series are both incorporated in the thickness of a horizontal sheet of dielectric material. The upper face of each of these transistors, which face is equipped with gate and emitter bonding terminals, points upward, whereas the lower face, equipped with a bonding terminal for the collector, points downward. To link these two transistors in series, it is therefore necessary to electrically link, for example the emitter of the transistor IGBT1 to the collector of the transistor IGBT2. This connection is called here an intermediate bond. Since the emitter of the transistor IGBT1 is on the upper face of the sheet, whereas the collector of the transistor IGBT2 is on the lower face of the sheet, this connection must cross the substrate. On the upper face of the sheet there therefore exist at least two different potentials Ve1 and Ve2 corresponding respectively to the potentials of the emitters of the transistors IGBT1 and IGBT2. Likewise, on the lower face of the sheet, there also exist two different potentials Vc1 and Vc2 corresponding respectively to the potentials of the collectors of the transistors IGBT1 and IGBT2. Since these two transistors are linked in series, the voltage Ve1 is equal to the voltage Vc2.
Due to the presence of various potentials on the same face of the substrate, the first and the second transistors have to be spaced apart by a minimum insulation distance. Below this minimum insulation distance there is a risk of a short-circuit being established between these two potentials, either by breakdown of the surrounding air, or by routing to the surface of the substrate. This insulation distance is therefore dependent both on the dielectric properties of air and on the value of the various potentials.
Hence, when one wishes to apply this technology to make power switching cells capable of working at voltages of several thousand volts, the minimum insulation distance to be complied with increases and the footprint of the switching cell likewise. This is a brake to the application of the teaching of the CPES article in the medium voltage sector.